A vehicle (e.g., an automobile) has a blower motor or a cooling fan motor used in a vehicular air conditioner of the vehicle. FIG. 5 shows a circuit configuration of a motor drive device for driving such motors. In the motor drive device, a series circuit including a P-channel metal oxide semiconductor field-effect transistor (MOSFET) 1 and a load 2 such as a DC motor is interposed between a positive terminal (+B) of a battery (power source) and ground. In other words, the load 2 is connected in a high side drive configuration in which the MOSFET 1 is provided in the high potential side. The DC motor rotates a blower fan (not shown) so that an air conditioner blows air out.
Further, another series circuit including a P-channel MOSFET 3, a resistor 4, a resistor 5, an N-channel MOSFET 6, and a resistor 7 is interposed between the positive terminal of the battery and the ground. The gate of the MOSFET 1 is connected to a connection point between the resistor 4 and the resistor 5. A pull-up resistor 8 is interposed between the gate of the MOSFET 1 and the positive terminal of the battery. The resistors 4,5,7 adjust a time constant required to drive the gate of the MOSFET 1.
A drive control circuit 9 receives a drive control signal Sd (i.e., a command for applying a voltage to the load 2) outputted from an air-conditioner Electronic Control Unit (ECU) that controls the air conditioner. The drive control signal Sd is a pulse-width modulation (PWM) signal having a carrier frequency of 5 kHz, for example. The drive control circuit 9 performs frequency-to-voltage conversion, for example, by means of a filter so that the PWM signal is converted to a voltage signal. The drive control circuit 9 creates a drive command signal based on the voltage signal and outputs the drive command signal to the gates of both the MOSFET 3 and the MOSFET 6, thereby turning on one of the MOSFET 3 and the MOSFET 6.
Specifically, when the MOSFET 3 is turned off and the MOSFET 6 is turned on, the gate of the MOSFET 1 changes to a low level so that the MOSFET 1 is turned on and the load 2 is energized. In contrast, when the MOSFET 3 is turned on and the MOSFET 6 is turned off, the gate of the MOSFET 1 changes to a high level so that the MOSFET 1 is turned off and the load 2 is not energized.
The circuit components except the MOSFET 1 and the load 2 construct a control integrated circuit (IC) 10. The control IC 10 and the MOSFET 1 are one-packaged as a motor drive IC 11. A motor drive device having a circuit configuration similar to the control IC 10 is disclosed in U.S. Pat. No. 6,891,342 corresponding to JP-A-2004-72977, for example.
In the motor drive device shown in FIG. 5, when an input terminal for applying the drive control signal Sd to the drive control circuit 9 becomes a high-impedance state, the control IC 10 stops its operation so that both the MOSFET 3 and the MOSFET 6 are turned off. In this case, the pull-up resistor 8 keeps the gate voltage of the MOSFET 1 at high level. Thus, a voltage between the gate and the source of the MOSFET 1 becomes 0 V, and the MOSFET 1 is turned off.
To check the quality of the MOSFET 1, a leak current flowing between the gate and the source of the MOSFET 1 is measured. Generally, the leak current measurement is performed before connecting the MOSFET 1 to the control IC 10, and then only the MOSFET 1 that passed the quality check is used for fabricating the motor drive IC 11.
However, when the MOSFET 1 is connected to the control IC 10 through a bonding wire, impact force may be applied to the MOSFET 1. As a result of the impact force, the MOSFET 1 may be broken so that the quality of the motor drive IC 11 may be decreased. In order to improve the quality of the motor drive IC 11, therefore, it is preferable that the quality of the MOSFET 1 is rechecked by re-measuring its leak current after connecting the MOSFET 1 to the control IC 10.
However, it is difficult to measure the leak current of the MOSFET 1 in such a manner, because the pull-up resistor 8 allows an electric current to flow into the gate of the MOSFET 1 and the electric current is smaller than the leak current.